The following invention relates to a method and apparatus for connecting two electrical devices and, in particular, semiconductor devices, by means of a bond wire connection that simulates the electrical properties of a transmission line and is thus free from inductive reactance at high frequencies.
Small integrated circuit elements such as microprocessors are typically connected to other semiconductos or circuit elements such as hybrid integrated circuits by wire bonding. The wire bonding technique utilizes a special machine to fuse extremely small diameter wires to the contact points or bond pads of these smaller IC chips. This method of physical interconnection of one semiconductor device to another is adequate where the upper limit of the frequency of the signal between the two devices is less than 10 megahertz. Bond wires, however, are inductive and at frequencies of around 100 megahertz, the inductance of the bond wire begins to create a reactive component in the connection of a magnitude which attenuates the signal level. This attenuation takes the form of a subtraction effect that occurs when a certain portion of the input frequency wave is reflected back to the source from the bond wire connection. At frequencies in the gigahertz range, the bond wire becomes an almost pure inductor which severely retards the incoming signal, and at a range of 10 to 30 gigahertz, there may be complete attenuation of the signal.
In hybrid IC chips, that is, relatively large chips which may include a ceramic substrate, high frequency RF signals are transmitted via a transmission line imprinted on the chip. Such transmission lines are described generally in a text, Gupta, Garg, and Bahl, "Microstrip Lines and Slot Lines" (Artec House, Inc., 1979). The transmission line on such circuits may take the form of a coplanar wave guide which includes a signal-carrying conductor flanked on opposite sides by a pair of ground plane conductors. All of the conductors extend substantially parallel to one another and are coplanar. It is at the interface between the transmission line of the hybrid IC chip and a smaller integrated circuit usually referred to as a die that the bond wire connection described above is made. The transmission line of the hybrid IC is fully capable of handling frequencies in the gigahertz range, but the inductive problem described above is encountered when attempting to connect the IC die to the hybrid IC transmission line.